Well all of them are actually Asians. In case nobody is dying their hair, all of them has hair in the light to dark brown range. If we go with diverse cast when it comes to race, I can imagine her as really dark-haired green eyed person. Michiru hair in the anime was somewhat inconsistent in color. Same with the manga and her hairlength varied a lot. So even when it comes to anime/manga universe, she was a bit odd.